Call _post_downgrade_### after downgrade migration is run

Fixes bug 1195470

Tests are currently only run after a upgrade during the database migration
unit tests and never during a downgrade. This change calls an optional
_post_downgrade_### method after running the downgrade migration to allow
it to be tested as well.

Change-Id: I592424b464b91c2be9e253402986e673ce33c8d0
Merge "Call _post_downgrade_### after downgrade migration is run"